Sorry, there's just... a lot of misunderstandings about Depletion Garden here. And some serious misassumptions.
AlinSabel has it right about Servants and magi.
First of all: There is absolutely no reason for a Servant to fall over dead (or be affected in any way, really) by Depletion Garden because they do NOT live off the prana in the ATMOSPHERE, they live off of prana from their Masters (and in Saber's case, her own reserves). In this way, that "breathing in space" analogy was working under the wrong premise since the start.
And Depletion Garden cannot suck prana out of living systems, only from the area within its effective radius (i.e. the air, etc.) (Also, where are you coming up with this "renders all magic useless"? There has to be a limit to speculation...) This would hinder certain Servants and magi, no doubt (i.e. if a Noble Phantasm / spell required prana from the atmosphere to activate
(I think Shiro said he saw Lancer sucking in prana from the atmosphere when preparing Gae Bolg, on the third day at the school)
), but this doesn't necessarily mean they're in serious trouble or at a severe disadvantage since it doesn't stop them in anyway to cast spells using their own prana reserves (or in Rin's case, toss a charged jewel).
And yes, Depletion Garden is canonical.
And no, Nanatsuya, Saber did not regain prana by
sleeping a lot. It just lowered the prana consumption to a minimum when she's sleeping. This makes the difference in cost and gain greater, and thus more sustainable. But the resting has no direct effect on the amount of prana gained (not that we know of).
Depletion Garden really isn't as great or useful as you might think it is (I blame the problem of Melty Blood giving us disillusions), since not that many mages (nay, I'd say none) appear in the Nasuverse that cast by drawing prana from the environment (Go ahead, give me one example other than Lancer. And that one is kind of iffy too since he doesn't really do it in other instances of activating Gae bolg) and it REALLY, REALLY doesn't "make Servants drop to the ground" (they're incapable of drawing prana from the environment even if they wanted to. That's why Caster had to resort to those
Hell, the Forest of Einnashe could do what Depletion Garden does just by being there.
It's only amazing in the sense that Satsuki could get a reality marble just by her potential and without any prior magical training or practice.
edit: another thing:
Still, if that stored power runs out, they're pretty screwed =P So a lasting battle would be out of the question.
Since most living systems (Magi, Servants, familiars) are incapable of drawing prana directly out of the environment anyways (at least not a significant amount in a short period of time), sucking the prana out of it doesn't REALLY make a difference. They all have enough stored energy to last A LONG TIME under normal circumstances so it doesn't make any sense to assume that as soon as DG hits the field, they'll run out of stored prana within the battle. As long as you don't try to, say, activate Excalibur (or something of the equivalent in prana usage) several times in the same battle (Actually I don't think anyone other than Dark Sakura has enough prana for that @_@), there's really no reason why you would run out of "stored power", as you say. (Servants are a special case, as I'd imagine they exhaust much more prana in battle since they require much more prana to just exist in the first place. But there' really no reason for a magus to run out of prana just from casting a few spells in battle)
But since Satsujinki isn't a magician, he will be almost unaffected by it^^
This conjecture makes even LESS sense by the "exhausting reserves" line of reasoning because as he ISN'T a magus, his prana reserves are SIGNIFICANTLY LESS than those of a magus. In any case the fight would almost certainly END with one side defeated/dead before anybody runs out of prana anyway, but that's besides the point. Depletion Garden doesn't really hinder anyone in terms of fighting, as long as they don't depend on the environment for prana (which nobody does)